In the context of this invention, a radiation-absorbing optical element is understood to refer both to a partially absorbing optical element and to one that absorbs, at least approximately, all the radiation. Partially absorbing optical elements are used, for instance, as optical filters for specifically controlled light reduction, in particular as color-neutral filters. Optical elements that absorb incident radiation almost entirely are also called black layers, and are used, for instance, in sensor technology in both the visible and near-infrared spectral ranges, in particular for pyroelectric sensors.
One method of manufacturing black layers consists of coating a surface with the aid of a very sooty flame of organic gas, such as acetylene. The thickness and the evenness of black layers created in this way, however, are often unsatisfactory. The surface, moreover, is strongly heated, which is particularly disadvantageous when delicate sensors are located behind the surface.
In another method a black layer is created from a porous metal layer by evaporating a metal such as silver in a low vacuum of about 1.33 mbar. Porous metal layers of this type, however, often only have limited long-time stability.
It is also known that metal layers can be converted through anodic oxidation or through plasma-chemical oxidation into black ceramic oxide layers. These, however, have undesirably high reflection values in the near-infrared spectral range, and can only be created on metallic or similarly temperature-stable and chemically resistant substrates.
The creation of a black player through the application of multiple layers of different compositions is known from FR-A-2 592 063. The first layer here is formed of a material from either group 4A or 5A of the periodic table of elements, followed by a nitride layer and a carbide layer of the same element. The sequence of layers is completed by a layer containing carbon.
The use of a black layer as a decorative coating is described in U.S. Pat. No. 4,634,635. This describes coating the surface of an ornament with SiC using a plasma-supported CVD process in which the surface is heated to a high temperature during the coating process. A method of this type is therefore not suitable for coating temperature-sensitive plastics.
DE 689 10731 T2 describes an absorptive coating created by cathode atomization (sputtering). Targets containing, for instance, a mixture of oxygen nitrides of aluminum and titanium are used. The manufacture of targets of this type, however, is expensive and involves considerable costs.
DE 102 41 708 A1 describes the use of a plasma ion source to generate a layer having a graduated refractive index that reduces the interface reflections of a plastic substrate.